1. Field of the Invention
This invention relates to devices and methods for preventing electrical disruption or destruction of electronic circuits and their components and, more particularly, to a novel apparatus and method for suppressing high-energy transients on electrical power lines, thus protecting electrical and electronic equipment.
2. The Prior Art
Over the last two decades the ways in which electrical energy may be purposefully applied in every day life have expanded greatly. Sophisticated computers organize, analyze, transmit, and store massive quantities of data; industrial machines are electronically controlled to delicate precision; radio and television stations automatically broadcast prerecorded programs; medical support systems, monitors, and instruments interpret, analyze, and report various data; and numerous home appliances provide necessities, comfort, and entertainment to families and individuals.
Because of electricity's wide application in our society, individuals, businesses, and governments have become increasingly dependent upon electrical energy and upon the continuous and proper functioning of electrical devices. Our society expends vast resources to acquire and operate useful electrical devices, and the temporary or permanent interruption of the services provided by such devices is both inconvenient and costly.
Assuming that an electrical circuit is well designed so as to minimize the possibility of circuit disruption under normal operating conditions, there are nonetheless some types of disturbances which may result in the temporary disruption, or even the destruction, of electronic circuit components such as large scale integrated circuits used in computers, microprocessors and the like. These disturbances result, in part, from the widespread use of electrical energy in our society, and may appropriately be considered as electrical pollution, roughly analogous, on an atomic level, to other forms of environmental pollution.
For convenience of discussion, electrical disturbances may be classified as one of three types: electrical noise, voltage sags or surges, and transients. Each of these types of disturbances is discussed briefly below.
Electrical noise, also sometimes referred to as "hash," results from random changes in voltage level. Voltage changes due to noise are not typically very large but they are fairly frequent, such that if a normal operational voltage level were plotted on a graph as a function of time, the voltage level would be represented by a "fuzzy" line instead of a sharp line. Noise in an electrical circuit may be caused by electromagnetic interference from nearby fluorescent lights, transformers, computers, car ignitions, radio and television transmissions, electrical storms or other sources of electromagnetic or radio-frequency signals.
Voltage sags or surges are a decrease or increase, respectively, of an AC voltage level for one or more voltage cycles. Sags or surges may originate from many sources, such as loose connections in a device, switches, power overloads, lightning, accidents, blackout and brownout corrections, short circuits, grounding, or operation of nearby electric motors and generators.
Transients are high-voltage pulses having an extremely fast rise time, typically on the order of a few nanoseconds, although some transients may last for a period of up to 10 milliseconds or even one-half of an AC voltage cycle. Typically they may reach a peak voltage of as high as 20,000 volts; however, transients which last for longer than 100 milliseconds, generally referred to as "spikes," do not typically exceed 6,000 peak volts. Transients may originate from many of the same sources mentioned above in connection with noise and voltage sags and surges. Some of the more common sources of transients are switches, short circuits, grounding, accidents, and electrical storms.
Each of these types of electrical disturbances may cause data errors, unscheduled downtime, circuit board failure, transistor failure, disruptive false commands, or loss of computer memory data. Extremely high voltages, mostly due to transients, may additionally cause insulation breakdown, "hot spot" melting of semiconductors, and the destruction of many delicate circuit components which in turn may necessitate costly service calls and repair time.
As the need for protecting electronic equipment from surges, spikes and transients (hereinafter collectively referred to as "transients") has increased, so has the need to set industry standards in this area. Organizations such as the Institute of Electrical and Electronics Engineers ("IEEE"), Underwriters Laboratories ("UL"), International Electrotechnical Commission ("IEC"), and the Federal Communications Commission ("FCC") have each promulgated standards concerning specifications of various simulated transient waveforms that equipment must survive. Of all the different standards, some of the most widely used and accepted are those promulgated by the IEEE.
The IEEE standard having the greatest applicability to the design and manufacture of devices for the protection of electronic equipment is IEEE Standard 587-1980 (hereinafter "IEEE-587"). IEEE-587 specifies the parameters of several simulated transient waveforms to be applied to the equipment under test in order to determine its ability to operate properly when subjected to severe transients.
One type of simulated transient specified in IEEE-587 is generally referred to as an impulse. An impulse waveform is unipolar and has an exponential rise and decay. FIG. 10 shows an oscilloscope trace of an impulse waveform. The waveform of FIG. 10 may be referred to as a 1.2.times.50 waveform. In this designation, which is common in the art, the first number indicates the rise time of the transient in microseconds, e.g., 1.2, and the second number indicates the duration to 50% decay of the transient in microseconds, e.g., 50.
The second type of simulated transient is generally referred to as an oscillatory or "ringwave" waveform. Oscillatory waveforms are characteristically sinusoidal waves having fast rise times to an initial peak which then decay. FIG. 11 is an oscilloscope trace of an oscillatory waveform which is characterized as 0.5/100 kHz waveform. The first number (0.5) indicates the rise time in microseconds. The second number (100 kHz) indicates the frequency of the oscillatory wave.
Under IEEE-587 different categories of simulated transients are used depending upon the installation of the equipment, as set forth below.